Recently with global environments as well as energies and resources, problems have come to the surface, and fuel cells now attract attention as one of energy supply systems for putting them in harmony with the industry. A fuel cell is a power generator wherein hydrogen-rich gas obtained by reformation of previously supplied hydrogen gas, or hydrocarbonaceous fuels such as natural gas, gasoline, butane gas and methanol is allowed to react electrochemically with atmospheric oxygen for giving direct access to electricity. A typical fuel cell using the hydrogen-rich gas comprises a reformer for steam reformation of hydrocarbonaceous fuel to generate hydrogen-rich gas, a fuel cell unit for generation of electricity, a converter for conversion of the resulting DC current into an AC current, etc.
Depending on the electrolyte used with the fuel cell unit, the reaction forms involved, etc., such a fuel cell is generally broken down into five types: a phosphoric acid type fuel cell (PAFC), a molten carbonate type fuel cell (MCFC), a solid electrolyte type fuel cell (SOFC), an alkali type fuel cell (AFC) and a solid polymer type fuel cell (PEFC). Of these, the solid polymer type fuel cell (PEFC), because of using a solid electrolyte, is conditionally more favorable than other fuel cells such as the phosphoric acid type fuel cell (PAFC), and the alkali type fuel cell (AFC).
Problems with the solid polymer type fuel cell (PEFC) are, however, that its electrode catalyst is poisoned by a slight amount of CO and its performance becomes much worse especially in high current density regions, because platinum is used as the catalyst and because of low operation temperature. This requires for the concentration of CO contained in the reformed gas (hydrogen-enriched gas) generated in the reformer to go down to about 10 ppm.
A hydrogen-purification membrane comprising a Pd alloy film has been developed as one of means for removing CO from the reformed gas to refine hydrogen. In principal, the Pd alloy film would be permeable to hydrogen alone, if it gets rid of pinholes, cracks or the like. Specifically, if the reformed gas side is placed at high temperature and pressure (for instance, 300° C. and 3 to 10 kg/cm2 (0.29 to 0.98 MPa)), hydrogen would pass to a low partial pressure-of-hydrogen side.
For a hydrogen-purification process using such a Pd alloy film as described above, the film must be thin because the rate of permeation of hydrogen is inversely proportional to film thickness. In view of mechanical strength, however, the Pd alloy film is only allowed to have a thickness of about 30 μm at most when used alone; when a Pd alloy film having a thickness of about a dozen or so μm is used, a support of porous structure has so far been placed on the low partial pressure-of-hydrogen side of the Pd alloy film. However, the Pd alloy film and the support must be separately attached to the reformer, giving rise to a problem in connection to the workability for obtaining good sealing, and a problem that friction occurs between the Pd alloy film and the support, rendering the durability of the Pd alloy film less than satisfactory.
To obviate the above problems, there has been developed a hydrogen-purification membrane wherein a Pd alloy film is formed directly on a support into an integrated Pd alloy film/support combination. For instance, there is available a hydrogen-purification membrane of the structure wherein a Pd alloy film is formed on one surface of a flat metal sheet, through-holes are then formed by etching through the flat metal sheet from its opposite surface, and a porous support substrate is finally applied onto the Pd alloy film to hold the Pd alloy film between them (JP(A)7-124453). There is also available a hydrogen-purification membrane fabricated by forming a Pd alloy film on a temporary support, then forming a resist pattern on the Pd alloy film, then forming a metal base film having micro-openings by electrolytic plating in such a way as to cover 30 to 95% of the Pd alloy film, and finally removing the temporary support (JP(A)2002-292259). Further, there is a hydrogen-purification membrane fabricated by placing a metal sheet on one surface of an electrically conductive substrate having through-holes, then copper plating another surface of the conductive substrate to form a copper plating layer in such a way as to fill in the through-holes, then removing the metal layer to form a Pd alloy film on the resulting surface, and finally removing the copper plating layer by selective etching (JP(A)2004-57866).
With the hydrogen-purification membrane set forth in JP(A)2004-57866, however, yields are low with difficulty in cutting down on fabrication costs, because upon the resist formation and the etching at the step of providing through-holes by etching of the flat metal sheet with the Pd alloy film formed on one surface from its back surface side, the Pd alloy film is susceptible to break down. Further, etching from one side of the flat metal sheet causes the opening diameter of each through-hole to become inevitably larger than the thickness of the flat metal sheet and the through-hole pitch to become wide, imposing limitations on the number of through-holes formed per unit area. To add to this, one-side etching of the flat metal sheet from its back surface side causes the opening diameter of each through-hole to become small on the Pd alloy film side and, hence, the area of the Pd alloy film contributing to the permeation of hydrogen to become small, eventuating in a drop of the hydrogen permeation efficiency of the membrane.
A problem with the hydrogen-purification membrane set forth in JP(A)2002-292259, on the other hand, is that much time is taken to form the metal base film on the Pd alloy film by electrolytic plating, and it is difficult to form a metal base film having sufficient strength and large thickness. There is another problem that the resist is likely to remain on micro-openings in the formed metal base film.
With the hydrogen-purification membrane disclosed in JP(A)2004-57866, voids not plated with copper often occur at the copper plating layer, especially at a deep site of each through-hole (the site to be later formed with the Pd alloy film) at the step of filling in the through-holes by copper plating. Such voids would otherwise be responsible for pinhole defects of the Pd alloy film. This causes a complex process control and offers obstacles to cutting down on fabrication costs.